Intermittent crash in /css/css-flexbox/flex-aspect-ratio-img-row-005.html

[jdm]

▶ CRASH [expected OK] /css/css-flexbox/flex-aspect-ratio-img-row-005.html
  │ 
  │ error: XDG_RUNTIME_DIR not set in the environment.
  │ libEGL warning: No hardware driver found, falling back to software rendering
  │ assertion failed: `(left == right)`
  │   left: `Some(Rect(NaNx150.0 at (8.0, 535.0)))`,
  │  right: `Some(Rect(NaNx150.0 at (8.0, 535.0)))` (thread WRRenderBackend#0, at /root/.cargo/git/checkouts/webrender-c3596abe1cf4f320/1175aca/webrender/src/batch.rs:277)
  │    0: servo::backtrace::print
  │    1: servo::main::{{closure}}
  │    2: std::panicking::rust_panic_with_hook
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libstd/panicking.rs:481
  │    3: rust_begin_unwind
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libstd/panicking.rs:385
  │    4: std::panicking::begin_panic_fmt
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libstd/panicking.rs:339
  │    5: webrender::batch::AlphaBatchList::set_params_and_get_batch
  │    6: webrender::batch::AlphaBatchBuilder::push_single_instance
  │    7: webrender::batch::BatchBuilder::add_segmented_prim_to_batch
  │    8: webrender::batch::BatchBuilder::add_prim_to_batch
  │    9: <webrender::render_target::ColorRenderTarget as webrender::render_target::RenderTarget>::build
  │   10: webrender::frame_builder::FrameBuilder::build
  │   11: webrender::render_backend::Document::build_frame
  │   12: webrender::render_backend::RenderBackend::update_document
  │   13: webrender::render_backend::RenderBackend::process_transaction
  │   14: webrender::render_backend::RenderBackend::run
  │   15: std::sys_common::backtrace::__rust_begin_short_backtrace

We end up with a NaN width despite the attempt in #26758 to avoid them. It's worth tracing backwards to see where it comes from.

[jdm]

https://github.com/servo/webrender/blob/f918028a729b18b2e2cfdb132efe39f63d159dcc/webrender/src/prim_store/mod.rs#L262-L276 is one place that might be worth investigating due to https://github.com/servo/webrender/blob/f918028a729b18b2e2cfdb132efe39f63d159dcc/webrender/src/prim_store/mod.rs#L301-L306. Also https://github.com/servo/webrender/blob/bf9e2103875012be97e4f56aa4af44aaec5165e0/webrender/src/util.rs#L137 and https://github.com/servo/webrender/blob/bf9e2103875012be97e4f56aa4af44aaec5165e0/webrender/src/util.rs#L169.